where \(x_{1}=p_{1}^{+}/P_{1}^{+}\)  and \(x_{2}=p_{2}^{-}/P_{2}^{-}\) 
denote light-cone momentum fractions. The TMD distribution \(f^{\gamma }(x_{1},\mathbf {p}_{1T}^{2})\) 
represents the usual unpolarized photon distribution, while \(h_{T}^{\gamma }(x_{1},\mathbf {p}_{1T}^{2})\) 
represents the distribution function of linearly-polarized photons
in an unpolarized nucleus [1]}. The cross section
(REF ) up to twist-2 can be expressed by
\(\sigma _{\mathrm {twist\:2}} & \approx & \frac{1}{32(2\pi )^{6}}\int d^{3}k_{1}d^{3}k_{2}\int dp_{1}^{+}d^{2}\mathbf {p}_{1T}dp_{2}^{-}d^{2}\mathbf {p}_{2T}\frac{1}{vE_{A1}E_{A2}E_{k1}E_{k2}}\nonumber \\& & \times \frac{1}{2x_{1}P_{1}^{+}}\frac{p_{1T}^{\sigma }p_{1T}^{\mu }}{M^{2}}h_{T}^{\gamma }(x_{1},\mathbf {p}_{1T}^{2})\frac{1}{2x_{2}P_{2}^{-}}\frac{p_{2T}^{\rho }p_{2T}^{\nu }}{M^{2}}h_{T}^{\gamma }(x_{2},\mathbf {p}_{2T}^{2})\nonumber \\& & \times L_{\mu \nu ;\sigma \rho }\left(p_{1},p_{2};p_{1},p_{2};k_{1},k_{2}\right)(2\pi )^{4}\delta ^{4}(p_{1}+p_{2}-k_{1}-k_{2})\) 
